This invention relates to a motorized power steering apparatus for automobiles or other vehicles. In the past, power steering apparatuses for automobiles were usually driven by hydraulic power generated by a pump. However, as hydraulic systems are bulky and heavy, in recent years, there has been a trend towards the use of electric motors to provide the drive force for power steering. A power steering apparatus which employs an electric motor to generate an auxiliary torque to assist the steering of the vehicle is referred to as a motorized power steering apparatus.
In a motorized power steering apparatus, a torque sensor measures the steering torque applied by the driver to a steering wheel. An electric motor which is connected to a suitable portion of the steering gear is then controlled in accordance with the measured torque to impart an auxiliary steering force to the steering gear. The auxiliary steering force lessens the force which need by applied to the steering wheel by the driver.
A conventional torque sensor has a torque-displacement converter which produces a displacement corresponding to the steering torque applied to the steering wheel. The torque sensor also includes a main potentiometer and an auxiliary potentiometer when generate output voltages which correspond to the displacement of the torque-displacement converter. Typically, each potentiometer generates a prescribed base voltage when the steering torque is zero. The output voltages of the potentiometers rise above the base voltage when a steering torque is applied to the steering wheel in one direction, and the output voltages fall below the base voltage when a steering torque is applied to the steering wheel in the opposite direction. The output voltages from the main potentiometer and the auxiliary potentiometer are compared with one another. If the difference between the two output voltages is within a prescribed range, it is determined that the torque sensor is functioning properly, and a voltage is applied to the motor which is proportional to the output voltage of the main potentiometer. If the difference between the two output voltages is outside the prescribed range, then it is determined that the torque sensor is malfunctioning, and no voltage is applied to the motor.
A mechanism for comparing the outputs of the two potentiometers functions as a fail-safe mechanism. Theoretically, it can prevent the motor from generating an auxiliary torque in the wrong direction should the torque sensor malfunction. However, if the time at which comparison of the output voltages takes place is too early, it is easy for the fail-safe mechanism to malfunction. On the other hand, if the time at which comparison takes place is too late, the danger is increased.